A radar device radiates radio waves from a point of measurement to space and receives a signal reflected from a target to thereby measure a distance, a direction, etc. from the point of measurement to the target. Development of a radar device capable of detecting not only a car but also a pedestrian or the like as a target by high-resolution measurement using short-wavelength radio waves such as microwaves or millimeter waves has advanced in recent years.
Generally, in a radar device, long-wavelength radio waves are so low in attenuation as to make detection of a distant place possible but are so low in resolution as to make accuracy of target detection low. On the contrary, short-wavelength radio waves are so high in attenuation as to make detection of a distant place difficult because of easy absorption or reflection by water vapor, cloud, rain, etc. contained in air but are so high in resolution as to make accuracy of target detection high. Radar devices disclosed in the following Non-Patent Literature and Patent Literature are known as conventional radar devices.
For example, Non-Patent Literature 1 has disclosed a radar device which scans an antenna mechanically and scans pulse waves or continuous waves electronically with a narrow-angle directional beam to thereby transmit radio waves and receive reflection waves reflected from a target. In the radar device according to Non-Patent Literature 1, the antenna scanning time is required for detecting the target because a single antenna is used for transmission/reception of radio waves.
For example, when a target moving at a high speed is to be detected, it is therefore difficult to detect the target while following the movement of the target because of the necessity of a lot of scans in accordance with required high-resolution measurement.
Non-Patent Literature 2 has disclosed a radar device in which a signal reflected from a target is received by a plurality of antennas disposed spatially and the phase of the received signal is measured without the necessity of a lot of scans so that an arrival angle is estimated with higher resolution than the directivity of each antenna though the beam directivity of each antenna is relatively wide.
According to the radar device of Non-Patent Literature 2, the arrival angle can be estimated by signal processing at thinned-out scanning intervals to thereby improve accuracy of target detection compared with the radar device of Non-Patent Literature 1. Moreover, even when the target is moving at a high speed, the arrival angle can be estimated following the movement of the target.
In Non-Patent literature 2, because a plurality of antennas are however used so that an RF (Radio Frequency) generator for amplifying a signal received by each antenna and down-converting the frequency of the signal to generate a baseband signal and a signal processor for applying A/D (Analog Digital) conversion to the generated baseband signal to calculate a desired arrival angle are provided in accordance with each antenna, the overall configuration of the receiver is complicated and a cost increase is brought.
Patent Literature 1 has disclosed a radar device and a target detecting method in which a switch for selecting one of antennas is provided so that a single transmitter and a single receiver can detect a target while the antenna receiving reflected waves from a target is sequentially selected by the switch. According to Patent Literature 1, simplification in configuration of the radar device can be attained because it is unnecessary to provide the RF generator and the signal processor in the Non-Patent Literature 2 in accordance with each antenna.
However, in Patent Literature 1, to correct a phase shift quantity generated by temporal change of operation in each of the transmitter and the receiver is unconsidered. The phase shift quantity generated by the temporal change is a variable phase shift quantity caused by temporal operation of a local oscillator provided in each of the transmitter and the receiver.
Accordingly, in the configuration of Patent Literature 1, a VCO (Voltage Controlled Oscillator) is connected only to the transmitter even if a reference signal is used in common for driving the local oscillators in the transmitter and the receiver. For this reason, PLL (Phase Locked Loop) circuits provided in the local oscillators respectively operate independently, so that a variable phase shift quantity is caused by temporal operation in between the transmitter and the receiver.
The arrival angle of the target is estimated on the condition that a phase difference dependent on the arrival angle of reflected waves from the target exists between antennas disposed in different positions. For this reason, when a phase shift quantity varying temporally is generated in each sequentially selected antenna and mixed with the signal received by the antenna, accuracy of phase detection according to each antenna deteriorates so that accuracy of target measurement deteriorates.
Moreover, Patent Literature 2 has disclosed a phase calibration device of an active phased array radar which switches a transmission signal as an input signal through a directional coupler and inputs the signal to the reception side as a reference signal in phase calibration.
However, in Patent Literature 2, because a process of distributing a part of the transmission signal through the directional coupler and inputting it to the reception side is performed by a switch, it is necessary to use the switch to notify the reception side of the output timing of the transmission signal whenever the transmission signal is transmitted. As a result, the process applied to the switch is complicated so that the overall configuration of the device is complicated.
However, in a configuration that a mechanism of inputting a part of the transmission signal to the reception side through the directional coupler in Patent Literature 2 is provided between the transmitter and the receiver in Patent Literature 1, a phase shift quantity temporally generated between the transmitter and the receiver can be measured by transmitting an attenuated signal of the transmission signal to the receiver even when the phase shift quantity is generated. Accordingly, there is an inference that deterioration of estimation accuracy of the arrival angle of the target can be reduced to some degree by correcting the measured phase shift quantity.
For example, when a calibration time is provided before a measurement time of a certain radar so that the phase shift quantity is measured in the calibration time, an actually measured phase difference between the transmitter and the receiver can be corrected in a measurement time of an actual radar device based on the phase shift quantity measured in the calibration time.